Just a couple of points that I keep meaning to add before another installment of the JHL build on

Alps Blue Velvet Potentiometers

Heatsinks

and finally transformers

Upgrading the Volume Potentiometer mounted on the PCB
There a number of areas on the PCB where there are size constraints and around the Volume Pot there is very little space. Reviewing the attached datasheet shows that this element would need to be an "off board" modification.

Increasing the sizes of the heatsinks
The form factor of the heatsinks used would allow an increase in size if required. The attached drawing shows that the width and depth are fixed and the height is changed for the application.

Transformer Selection
The choice of 25 or 50VA transformer was based apon the regulation of the size of transformer, broadly speaking the larger the transformer the better the regulation. Further information can be found on the attached datasheet.

All of the above information has been taken from Rapid Electronics here in the UK. Over the past 20 years without exception I have always found them profession, helpful and a great company to deal with. (also their catalogue and website are much simplifier and smaller than others we can mention)

With all the future modifications I have tried to use easy to find worldwide sourced components from the big companies.

Comments
The performance of the JHL kit was not as expected or designed by JLH and having serious reservation with regard to the PCB layout proof was required that the project should continue. The work done for this post was to improve the amplifier performance at little or no cost and to check the PCB for function.

Limitations in testing
It must be made very clear that the test regime for the amplifier is currently very basic with a number of omissions, these are

Poor input test signal quality at low frequencies

The full bandwidth has not been tested

The input signal has been limited to a small value of 200mV peak to peak

Testing at 20, 200, 2k and 20kHz would be more useful and the design improves

Both sine and square testing at the above frequencies

Steps are now being taken to improve the testing setup by serval orders of magnitude.

Stage 1
These simple changes will improve the performance of the board considerably.

C23, C24 Require inserting with the correct polarity. The negative electrolytic capacitor terminals should face the centre of the board. If the board has been powered up can I suggest this component is replaced.

Remove the 8 (0.1uf) capacitors shown in photo 3

Stage 2
With a small number of replacement components the design takes another step forward. Using picture 3 as our guide

Implement Stage 1

Change 2k2 for 3k3 resistors (2 off)

Remove the two 10k variable resistors and link as required

Replace 4k7 with 10k (2 off)

Replace 220k with 47k (2 off)

Replace the Zener Diode with 3 x 1N4148 in series (6 off) - Note the polarity is opposite to that of the zener.

Please note that there is a larger offset voltage which cannot be removed with this method in the order of ±100mV.

Impulse Testing
Below are a series of waveforms to test the design under ideal conditions by applying an impulse to the input, in this case a square wave and the output is displayed on an oscilloscope. The oscilloscope has two channels CH1 and CH2 which have the same functionality. The X10 (Times 10 or multiplied by 10) versions show an expanded X axis of the graph to quickly show the effective rise time of the amplifier.

In the following tests CH1 (Top of the screen) displays the input waveform from the signal generator and CH2 (Bottom of the screen) displays the output of the amplifier. In a perfect world both signals should look the same and the rise time of each signal should match.

There are 6 pictures, 3 sets of two for the following frequencies

100Hz Picture 4 and 5

1HKz Picture 6 and 7

10kHz Picture 8 and 9

On the right hand side of each picture there are a number of measurements taken by the scope which are helpful to understand what is happening..

The first is the frequency in Hz of the input signal

The second is the rise time of the orignal signal on Channel 1 CH1

The third is the rise time of the output of the amplifier on Channel 2 CH2

In Conclusion
These small changes have made a considerable improvement to the JHL kit with respect to the rise time of the headphone amplifier and therefore bandwidth. So in real terms there should be more detail and life in the top end..

Next Step
To fully implement the original JLH design with the correct capacitor values and change the output transistors to the original 2N492x. These components are currently on order.. In real terms these are small changes but personally I am very interested to see the difference they will make.

After modding to stage 1, i don't hear the différence but after changing R to reduce the gain and well ordered the offset, Yes it's seems to me that the sound is better, more bass, voice are better defined. Thanks for you help.
- Transformer: i'm going to by a 50VA Rcore 2x 12v because mine is too hot
-change the n4007 by BYW 98-200, some blackgate capacitors and two muse on the output
-For the ALPS, it must be deported on the panel, because it's too small for implant it on the PCB
Regards

Want to thank Miles for sharing his journey with such thoughtfulness, detail and clarity. I'm looking for a good headphone amp to build and the JLH design may be it, or even the JHL version, so I'm looking forward to see if Miles can redeem that board

May I throw in my 2c here and suggest instead of an Alps pot that the Lightspeed attenuator be considered? I've found LDRs to be so much more transparent sounding than mechanical pots, stepped attenuators or TVCs.

This week I have completed another two configurations of the JHL->JLH board with testing and I'm pleased to report that we have progress. It should be clearly understood the original design is now antique in the terms of electronics and audio design but has stood the test of time very well. However modern sources eg CD players are designed for a 2Volt peak to peak output (1.4VRMS) with programme content averaging 300-500mV, JLH designed this headphone amplifier with a 0.707VRMS or 1Volt peak to peak 100-200mV.

There is a mistake on post #147
To Done List - Point 4. Reduced the gain of the amplifier for a 2Vrms input rather than the older 0.707Vrms standard
Should read - Reduced the gain of the amplifier for a 2Vpeak to peak input rather than the older 0.707Vrms standard

To Done List

JHL Simplified with correct component values - Done

Reduced the gain of the amplifier for a 2Vpeak to peak input rather than the older 0.707Vrms standard

Investigate JLH original 2N492x output transistor

Investigate the ability of the power supply to perform and will this require an output protection module required ? - This month

Are larger heatsinks required ? - Due to the reduction in gain this may not be an issue

Draw a logical and simple schematic - When the circuit is finalised

Investigate Lightspeed attenuator

fj12centauro - I look forward to your opinion when all is complete. Can I suggest waiting before you buy a new transformer as there is a pending question on the power supply.

Bernie7 - Thank you for your kind words. I have added the attenuator to the growing list.

jambul - Interesting question! By performance do you mean bandwidth/frequency response? The new testing seems to indicate that the current design is good, but I want to investigate why JLH chose the 2N4922 transistor. He did everything for a reason and I need to understand this decision, however I think it is due to current gain of the final stage. More on this very soon.

fj12centauro - Replacing the 1N4007 is interesting. After much research the BYW98-200 was made by ST but seems to have been obsoleted, the datasheet is easily found but the device is missing from the ST website.

Bernie7 - The attenuator uses a Silonex NSL-32SR2 Optocouplers. This seems to be a VERY cheap and interesting way to control the volume, the devices are inexpensive and available from Farnel ..

Testing
The testing rig used is really starting to show it's age and limitations now the design is starting to develop, as mentioned previously the signal generator was made at college and the scope has an issue with a DC offset on channel 2. Also it is now becoming important to measure the bandwidth and THD figure of the headphone amplifier as these were the tenants on which the design was conceived.

Dummy Load
All the testing completed so far has used some pretty horrid (Technical term) wirewound 100ohm 1 watt resistors similar to that used by JLH in the original articles. This part of the testing needs to be improved across the various values in the headphone market today

10 ohm

68 ohms

330 ohms

680 ohms

10 ohm + 0.22uf

68 ohms + 0.22uf

330 ohms + 0.22uf

680 ohms + 0.22uf

The resistors used will be the standard 6watt wirewound variant commonly found in distribution. JLH also tested with a resistive and capacitor load of 0.22uF this will give a worse case scenario rather than pretty test figures.

The components that are being selected for the design and testing are hopefully available worldwide so that the work can be replicated in the future regardless of location.

After a bit of testing, I've settled on increasing the gain resistor, R5 on the JLH schema, to 2K. This reduces the overall gain. The input capacitor has been increased to it's proper value of 1uf. C2 has been replaced with a low ESR type, and C4 has been increased to 220uf low ESR (both numbered by JLH).
Using a 15-0-15 AC 30VA transformer the regulator heatsinks are at about 60C (ambient is 23C), the TIP41s are at about 50C. Offsets are varying between +/- 5mV.
The board is on final soak test before being boxed and classed as 'done'. It sounds very good, and a bargain for the cost.
Next on the chopping board is the Panda headphone amp. Damn these cheap kits, I've got the 'bug' I thought I'd cured years ago. I've a nice workshop with decent equipment now though! (Question: Should all this equipment and components etc. be added on to the cost of the kits? If so, for me, they're crap value for money )

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